This invention relates to a knee-joint endoprosthesis comprising a femoral part, a meniscus element, and a tibial part, wherein the femoral part and the tibial part have bearing surfaces which contact associated bearing surfaces on the meniscus element.
U.S. Pat. No. 4,085,466 discloses a knee-joint endoprosthesis of this type, in which two artificial meniscus elements are disposed between spherically curved bearing surfaces on the femoral part and substantially planar bearing surfaces on the tibial part. The curved bearing surfaces of the femoral part and of the meniscus elements are expensive to produce, and due to production tolerances, it is hard to avoid localized high surface pressures per unit area. To facilitate some kind of equalization, the meniscus element is formed of flexible plastic. Slight departures from the geometry of the bearing surfaces and the resulting pressures per unit area and peak stresses can produce comparatively rapid wear and damage, substantially reducing the useful life of the prosthesis. It is known that, under compressive loading, plastics tend to develop permanent deformation, and the geometry of the originally complementary bearing surfaces changes. Making the meniscus elements from a flexible plastic, such as polyethylene for example, leads in practice to permanent deformation of the meniscus element after a period of time, so that the originally complementary configuration of the bearing surfaces of the femoral part and the meniscus element can no longer be assured, and the functionality of the endoprosthesis may be jeopardized. The meniscus element not only can execute linear movements on the planar bearing surface of the tibial part, but also can rotate about the axis which is perpendicular to the planar bearing surface of the tibial part and extends through the center of the spherically curved bearing surface of the femoral part. Therefore, additional abutments are provided to limit the angular range of rotation of the femoral part. This creates the danger that, due to linear or even point contact, high pressures per unit area, undesirably high peak stresses and damage may occur.
German Published Application No. DE 35 28 204 discloses a knee-joint endoprosthesis in which the femoral part and the tibial part each have a guiding means. These guiding means preferably take the form of a groove and projection cooperating with one another to enable flexional positioning in the sagittal plane. The guiding means are configured such that a mutual rotation between the femoral part and the tibial part is permitted in order thus to obtain an additional degree of freedom corresponding to the natural knee joint. Even in this knee-joint endoprosthesis the bearing surfaces of the femoral part are configured as spherically curved surfaces which in turn are mounted in contact with correspondingly curved surfaces of the tibial part. On the other hand, an additional meniscus element is not provided, so that the movements only roughly correspond to a natural knee joint.
U.S. Pat. No. 4,224,696 discloses an endoprosthesis having a tibial part with a curved bearing surface for the plastic meniscus element, the center of this curved bearing surface lies near the femoral part. The bearing surfaces of the femoral part and tibial part are substantially coaxial with one another and are configured to correspond approximately to the surface configurations of the natural parts of the knee joint. Only a single plastic meniscus element is provided having a convexly curved bearing surface in contact with the mating, concavely curved bearing surface of the single tibial part. The cost of manufacturing such a knee-joint endoprosthesis is considerable, and in practice it is impossible to avoid high pressures per unit area due to line contact.